I asked my Japanese sensei about that. He said it's only an expression and not really a word with meaning. The closest expression in English that is similar with that one is: "Oh!" Like when you're saying: "Oh! There she is". Hope that helps. Happy learning!

I am not a fluent speaker in anyway, but I researched the symbols you linked and this is what I found: Given that there is only slight differences in the way shi ( ｼ ) and tsu ( ﾂ ) are written, the word you saw could be said "atsu" or "ashi" The only translation I could find for "atsu" was through an online romaji translator which said it meant "administrator". My personal dictionary did not have any listing for the word at all. The same online translator as well as my personal dictionary gave me this list for possible meanings to "Ashi": foot, reed, tentacle, leg, shank, pes, limb, step, pace, footpace, footstep, tread, paw, clutch, spiculum, pediment, skirt, adhesiveness, clamminess, glutinosity, stick, stickiness. Hopefully one of them will fit the context of your friend's avatar :)