The headword (accusative case) is presumably extracted from the quotation given, the only other attestation of this adjective.
[1] Greek Anthology 6.192.5 (Archias); cf. beta 365, phi 192. The item in question is a fishing-rod; cf. generally delta 1359. On this epigram, a fisherman's dedication, see Gow and Page (vol. I, 404-407); (vol. II, 438-439); and further excerpts from this epigram at gamma 54 and iota 68. As Gow and Page note (vol. II, 439), the tri- here is simply an intensifier; the fishing-rod is a very long reed.