Lexical Definition

‎ὑποζώννυμι‎ hypozṓnnymi (hoop-od-zone'-noo-mee) to undergird ^G:V
from ‎ὑπό‎ and ‎ζώννυμι‎

undergirt.

• to gird under, i.e. frap (a vessel with cables across the keel, sides and deck)

Abbott-Smith

** ὑπο-ζώννυμι [in LXX: II Mac 3:19 * ;]
to undergird (Hdt., al.; ὑπὸ τ. μάστους, II Mac, l.c.); of a ship, to undergird or frap: Ac 27:17 (v. DB, ext. 367a).†

Abbott-Smith References

Liddell-Scott-Jones

ὑποζώννυμι

(infinitive ὑποζωνύναι in [Refs] and ὑποζωννύω [Refs 2nd c.BC+], undergird, τοὺς ἵππους ῥυτῆρσι [Refs 1st c.AD+]; ὁ ὑπεζωκὼς τὰς πλευράς (i.e. ὑμήν), or absolutely ὁ ὑπεζωκώς, the pleura, [Refs 2nd c.AD+] is expressed in [Refs 2nd c.AD+]; ὑπεζωκότες foetal membranes, [Refs 2nd c.AD+]; lining of the intestines, [Refs 4th c.AD+] —passive, especially in perfect participle, ζειρὰς ὑπεζωμένοι (variant {-ζωσμ-}) girt with ζειραί (which see), [Refs 5th c.BC+] (οι) girt up, [Refs 5th c.BC+] —especially,

__II brace a ship, so as to make her seaworthy [NT+2nd c.BC+]
__III ὑπεζῶσθαι· τὸ εἰς ἄνδρας ἐλθεῖν, Φιλητᾶς, [Refs 5th c.AD+] (probably = come to man's estate).

_{LSJ Scripture Refs: Acts.27.17}