mid-14c., "to understand, take into the mind, grasp by understanding," late 14c., "to take in, include;" from Latin comprehendere "to take together, to unite; include; seize" (of catching fire or the arrest of criminals); also "to comprehend, perceive" (to seize or take in the mind), from com "with, together," here probably "completely" (see com-) + prehendere "to catch hold of, seize."
The (partial) range of senses in Latin prehendere was "to lay hold of, to grasp, snatch, seize, catch; occupy violently; take by surprise, catch in the act; to reach, arrive at;" of trees, "to take root;" of the mind, "to seize, apprehend, comprehend," though this last sense is marked "very rare" in Lewis & Short.
It is a compound of prae- "before" (see pre-) + -hendere, found only in compounds, from PIE root *ghend- "to seize, take." De Vaan regards the compound as Proto-Italic. Related: Comprehended; comprehending.
Compare the sense development in German begriefen, literally "to seize," but, through the writings of the 14c. mystics, "to seize with the mind, to comprehend."