truant and trout

The earliest meaning of truant in English is "one who begs without justification" (OED). I find this interesting, as I don't think we have a single lexical item for this anymore.

It is borrowed from Old French truant, probably borrowed from Celtic, cf Welsh truan "wretched", Scots Gaelic truagh "wretched, pitiful". MacBain's derives this from Proto-Indo-European *streig- "to stroke, rub, press". The AHD and Pokorny derive it from an extended form of *terh₁- "to rub".

An extended form of *terh₁-, something like *troh₁-g-, became Greek τρώγω "to gnaw" and τρώκτης "gnawer" also "a sea-fish with sharp teeth". This was borrowed into Late Latin as tructus and into Old English as truht, becoming modern trout.

pregnant

pregnant is from French prégnant "full of meaning", from Latin praegnant- "with child, pregnant, swollen". This is related to praegnāre "to be pregnant".

But there's another pregnant meaning "Of an argument, proof, piece of evidence, etc.: compelling, cogent, convincing; clear, obvious".

She cold was, and withouten sentement… And this was hym a pregnant [v.rr. preignant, prygnant; preuaunt] argument That she was forth out of this world agon. - Chaucer Troilus & Criseyde

This is from French pregnant "compelling, pressing" from prembre "to press" from Latin premere "to press". From an early date it was influenced in spelling and meaning by the other pregnant.

Rosenberg's style hangs on his prose like a maternity dress, concealing a not particularly pregnant argument. - British Journal of the Philosophy of Science 34 412, 1983