I found the exclamation bombax! in Plautus' Pseudolus (Pl. Ps. 1.3.131), where note 19 specifies it is a Greek loanword (βομβάξ in fact) used as an interjection of contempt. This agrees with what is stated in this 18th century edition.
However, here bombax is said to mean "Splendid!", "Marvelous!", while Lewis and Short states it is an exclamation of real or affected surprise.
Finally, according to the Italian Olivetti Latin Dictionary it can be translated as accidenti!, which in fact has both nuances of meaning.
So what is it, and would 200 AD Romans still use this exclamation?