Why can someone be intrepid but not trepid ?
The Free Dictionary and Merriam-Webster both consider trepid to be a real word, but my computer’s little spell-checker program does not recognize it as such.
Why can’t someone be trepid, despite the fact that one can feel trepidation?
Etymonline says that trepidus is Latin for scared; and trepid does/did appear to be a word, as per Merriam-Webster. Google Ngrams tell us that trepid has decreased in use ever since the 1800s. So trepid was a word at one point, and still is (technically), but its popularity is continually decreasing; essentially, it’s obsolete. For some reason, trepidation is still widely used, but that’s one of the quirks of the English language; words are always going “out of style” or becoming obsolete.