Wiktionary
vb. (context archaic English) (en-third-person singular of: think)
Usage examples of "thinketh".
Oh, shall I ever learn the sweet charity that thinketh no evil, and believeth all things?
The grooms and the warder at the great gate she will trust, but it is the postern she feareth, because she thinketh an enemy might be secretly admitted there.
Howbeit I had no time then to rest my selfe, for the Mother of the boy, weeping and lamenting for his death, attired in mourning vesture, tare her haire and beat her breast, and came presently into the stable, saying, Is it reason that this carelesse beast should do nothing all day but hold his head in the manger, filling and belling his guts with meat without compassion of my great miserie, or remembrance of the pittiful death of his slaine Master : and contemning my age and infirmity, thinketh that I am unable to revenge his mischiefs, moreover he would perswade me, that he were not culpable.