Quick Definition
ῥώννυμι v. I strengthen, farewell
Strong's Definition
prolongation from rhoomai (to dart; probably akin to 4506); to strengthen, i.e. (impersonal passive) have health (as a parting exclamation, good-bye)
KJV Usage: farewell
UBS Dictionary of the Greek New Testament
verb
23.133 · (ῥώννυμαι[a]) · Physiological Processes and States · Health, Vigor, Strength
to fare well (derivative of a base meaning literally ‘to be strong,’ ‘to be well,’ but used in the NT only as a formula for the ending of letters) to be well and to fare well The occurrence of ῥώννυμαι as an epistolary formula may be understood in two different senses: either as a sincere expression of the wish that the person involved may fare well or merely as a device to indicate that the letter is complete. This second sense of ῥώννυμαι is treated in 33.24.
33.24 · (ῥώννυμαι[b]) · Communication · Discourse Types
goodbye to end a letter by means of the expression ἔρρωσθε (ἐρρώσσω in some manuscripts), always having a favorable connotation Since in ACT 15:29 ἔρρωσθε ends a letter, it may be necessary in some languages to employ a typical letter-ending formula which will at the same time be favorable, for example, ‘with best wishes’ or ‘with sincere greetings.’ It would be quite wrong to use literally an expression such as ‘goodbye’ if this would be employed only or principally in direct address rather than at the end of a letter. For another interpretation of ἔρρωσθε in ACT 15:29, see 23.133.
1 The analysis of ῥώννυμαι in its epistolary usage is similar to the use of the term dear in English, used at the beginning of a letter. It may actually represent an affectionate greeting, but it is more likely to occur simply as a greeting formula.
UBS Dictionary of the Greek New Testament © United Bible Societies, licensed CC BY-SA 4.0.
Thayer's Greek Lexicon
ῥώννυμι : to make strong, to strengthen; pf. pass. ἔρρω- | letter, ἔρρωσο, farewell: Acts xxiii. 80 [RG]; ἔρρωσθε,
Mounce Concise Greek Dictionary
ῥώννυμι rhōnnymi 1x
to strengthen, render firm; to be well, enjoy firm health; at the end of letters, like the Latin vale, farewell, Acts 15:29*
Abbott-Smith Greek Lexicon
ῥώννυμι
[in LXX: II Mac 9:20 11:21, 28, 33; III Mac6 * ;]
to strengthen; most freq. in pf., ἔρρωμαι, to put forth strength, be strong, hence, often in imperat., ἔρρωσο, ἔρρωσθε, farewell (Lat. vale): Ac 15:29 23:30 Rec., R, mg.†
Moulton & Milligan — Vocabulary of the Greek NT
ῥώννυμι. "Eppwco, ἔρρωσθε (wale, walete), are regular closing formulae (as in Ac 1539) in both private and official letters. Naturally they are much varied by the addition of terms of endearment and otherwise, particularly during the second and third centuries A.D. The following exx. must suffice : P Tebt II. 315° (ii/A.p.) ἔρρωσό μοι, τιμιώτατε, P Hamb I. , ῥωννυμι 5411 15 (ii/iii A.D.) ἔρρωσό μοι πολλοῖς χρόνοις ὑγιαίνων μετὰ καὶ τῶν σῶν, P Oxy XII. 158615 (early iii/A.D.) ἐρρῶσθ(αι) εὔχομί(αι) [π]ανοικεί, 26. I. 12213 (ili/iv A.D.) ἐρ]ρῶσθαί σε, κύριέ μου ἄδελφε, πολλοῖς χρόνοις καὶ προκόπτειν εὔχομαι. Many other exx. will be found in Exler £f7stolography p- 74 ff. For the verb = ‘‘have strength” of persons cf. PSI V. 495% (B.c. 258-7) βουλόμεθα yap σε τῶι τε σώματι ἐρρῶσθ[αι, and of trees cf. CP Herm 1. 281! φοίνικες ἄλλοι ἐρ[ρωἸμένοι. The subst. ῥῶσις is used in connexion with praying for “strength” for anyone, e.g. OG/S 2064 εὐξάμενος δῶσιν kal τέκνοις καὶ γαμετῇ. See also in application to the body Vett. Val. p. 16015.
Liddell-Scott-Jones — Greek-English Lexicon
ῥώννυμι (rhōnnumi) — farewell. ῥώννῡμι, [Refs 5th c.BC+]; ῥωννύω, [Refs 5th c.BC+]future ῥώσω (ἐπιρ-) [Refs 1st c.AD+]: aorist ἔρρωσα [Refs 5th c.BC+]:—passive, ῥώννῠμαι [Refs 1st c.AD+]: future ῥωσθήσομαι [Refs 2nd c.AD+]: aorist ἐρρώσθην [Refs 5th c.BC+]perfect ἔρρωμαι (see. below):—strengthen, τροφὴ ῥώννυσι [Refs 5th c.BC+]; ῥ. τὰν ψυχὰν ποτ᾽ (πρὸς) ἀλκάν [Refs 5th c.BC+]
II. mostly in perfect passive (with present sense) ἔρρωμαι, and pluperfect ἐρρώμην (as imperfect):—have strength or might, γέροντές ἐσμεν κοὐδαμῶς ἐρρώμεθα [Refs 5th c.BC+]
II.2. to be eager, enthusiastic, glossed by προθυμεῖσθαι, [Refs 5th c.BC+]: with infinitive, to be eager to do, ἔρρωτο πᾶς ξυνεπιλαμβάνειν [Refs 5th c.BC+]
II.3. to be in good health, [Refs 5th c.BC+], farewell, the usual way of ending a letter, as in [Refs 5th c.BC+], and at the close of [Refs 5th c.BC+]; εἰ ἔρρωσθε εὖ ἂν ἔχοι [Refs 2nd c.BC+]
II.4. frequently in participle ἐρρωμένος (which see).