Hapax

It's difficult to ascertain the meaning of a hapax legomenon.

Source: tatoeba (3226335)

The word 'Flother' only occurs once in Old English, thus making it a hapax legemenon.

Source: tatoeba (8466157)

The Assyrian student, moreover, has an advantage which the Hebrew student has not; he possesses dictionaries and vocabularies compiled by the Assyro-Babylonians themselves, and these frequently throw light on a word which otherwise would be a “hapax legomenon.”

Source: tatoeba (12172501)

Never build a doctrine on or draw a teaching from an unclear or debated hapax.

Source: wiktionary