Why can’t one be “trepid”?

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.

One can be trepid, but it's archaic. English has a lot of obsolete words like this, where a derivative is in common use but the root has fallen out of favour.

One can. At least, one could in 1859 when Thackeray wrote in 'The Virginians', The poor little trepid creature, panting and helpless under the great eyes. It is, however, as Optimal Cynic suggests, rare.

With some of these obsolete words, I think it's worth a shot to at least try to bring them back into use. I think trepid is the sort of word that sounds nice - personal opinion, and it would be nice to start using it. Enough people start to put it in, and and an Ngram in 2016 might show an upward graph.

I'm told that 'thrice' is obsolete in the UK. It's apparently treated like 'thou' and 'trice.' I'm from India, and we use it at the drop of a hat.