Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
1. phū, n. indecl., or phun, = φοῦ, a kind of valerian, Plin. 12, 12, 26, § 45; Scrib. Comp. 176; 177.
2. phu or fu, interj., foh! fugh! phu in malam crucem, Plaut. Ps. 5, 2, 5 (Ritschl, fu); cf. Charis. p. 213 P.; Prisc. p. 1024 ib.