by 1812, "pertaining to mechanical forces not in equilibrium, pertaining to force producing motion" (the opposite of static), from French dynamique introduced by German mathematician Gottfried Leibnitz (1646-1716) in 1691 from Greek dynamikos "powerful," from dynamis "power," from dynasthai "to be able, to have power, be strong enough," which is of unknown origin. The figurative sense of "active, potent, effective, energetic" is from 1856 (in Emerson). Related: Dynamically.
early 15c., pois, "weight, quality of being heavy," later "significance, importance" (mid-15c.), from Old French pois "weight, balance, consideration" (12c., Modern French poids, with -d- added 16c. on supposed derivation from Latin pondus "weight"), from Medieval Latin pesum "weight," from Latin pensum "something weighted or weighed," (source of Provençal and Catalan pes, Spanish, Portuguese, Italian peso), noun use of neuter past participle of pendere "to hang, cause to hang; weigh" (from PIE root *(s)pen- "to draw, stretch, spin").
Original senses are obsolete. The figurative sense (in reference to abstract things) of "steadiness, balance, equilibrium, composure" is recorded from 1640s, from the sense of "a state of of being equally weighted on either side" (1550s). The meaning "way in which the body is carried" is from 1770.