That they strive not about words (μη λογομαχειν). Word apparently coined by Paul from λογομαχια (1 Timothy 6:4 which see), a back formation in that case. A mere war of words displeases Paul. (Titus 3:9).

Useful (χρησιμον). Late and rare word from χραομα, here only in N.T.

To the subverting (επ καταστροφη). Old word (from καταστρεφω, to turn down or over), here only in N.T. (except 2 Peter 2:6 in some MSS., not in Westcott and Hort)." Because of the overthrow" (result επ, not aim), useless for this reason. Such war of words merely upsets the hearers.