The headword is an adjective in the masculine nominative singular; see generally LSJ s.v., and cf. mu 34 (gloss), and pi 167 (gloss).
The source of the entry is uncertain. There is a secure attestation of the headword in Menander, Epitrepontes 385 Sandbach (like a shopper at the marketplace, Syriscus squeezes a toy rooster among the trinkets and decides that it is tough); but note also e.g. the variant reading strifnoi\ ge/rontes (for striptoi/) preserved by Erotian for Aristophanes, Acharnians 180.
[1] The glosses are the same grammatical form as the lemma. The headword is identically glossed by Photius' Lexicon. Hesychius adds the additional gloss pukno/s (close, compact, stocky, sturdy); see generally LSJ s.v., and cf. pi 3149, and pi 3153. The first gloss only is given by the Synagoge and Lexica Segueriana 372.1. Also see Etymologicum Magnum 730.24 (Kallierges) s.v. strifno/n and Timaeus' Platonic Lexicon s.v. strufno/n (sour, harsh, astringent). [In her critical apparatus Adler reports that ms A spells the first gloss sfrikto/s, and ms V reads sfikto/s.]