From BiblePortal Wikipedia

Vine's Expository Dictionary of NT Words [1]

1: Πού (Strong'S #4225 — Adverb — pou — poo )

a particle, signifies "somewhere" in  Hebrews 2:6;  4:4 , RV (AV, "in a certain place"); the writer avoids mentioning the place to add stress to his testimony. See Haply , No. 5, VERILY.

Webster's Dictionary [2]

(adv.) In some place unknown or not specified; in one place or another.