Today I leaned a new word while reading John J. Nance’s Lockout. Wiktionary defines it as:

(archaic) Lacking confidence in others; distrustful.
Lacking self-confidence; timid; modest

And Microsoft Bookshelf 1991 for DOS on CD-ROM defined it on my retro computer tower next to me as:

diffident adj. Hesitant to assert oneself from a lack of self-confidence; timid. [ME < Lat. diffidens, pr.part. of diffidere, to mistrust : dis-, not + fidere, to trust.]

The pilot in the book was using the archiaic, mistrusting definition. It fits, given the archaic hardware I used to access the latter definition for pointless fun.

I don’t think I’ve ever heard the word before; it sounds more like a brand of toothpaste. Diffident: For when you don’t trust other brands.