In modern use restricted to the loading of ships; past participle laden was active in the language longer, but in 20c. was displaced by loaded (but a distinct word in the literal sense would be useful) except in particular phrases. Compare Lading.
"left-hand side of a ship" (to a person on board and facing the bow), 1580s, alteration of Middle English ladde-borde (c. 1300), perhaps literally "the loading side," if this was the side on which goods were loaded onto a ship, from laden "to load" (see lade) + bord "ship's side" (see board (n.2)).
Altered 16c. by influence of starboard, then, to avoid confusion of similar-sounding words, it was largely replaced by the specialized sense of port (n.4). The Old English term for it was bæcbord, literally "back board" (see starboard), a term which remains in the other Germanic tongues.
c. 1200, lode, lade "that which is laid upon a person or beast, burden," a sense extension from Old English lad "a way, a course, a carrying; a street, watercourse; maintenance, support," from Proto-Germanic *laitho (source also of Old High German leita, German leite, Old Norse leið "way, road, course"), from PIE root *leit- (2) "to go forth" (see lead (v.1)).
It seems to have expanded its range of senses in early Middle English, supplanting words based on lade (v.), to which it is not etymologically connected. The older senses went with the spelling lode (q.v.). The spelling is modern. Meaning "amount customarily loaded at one time" is from c. 1300; meaning "a quantity of strong drink taken" is from 1590s. Meaning "the charge of a firearm" is from 1690s.
Meaning "a great amount or number" (often loads) is from c.1600. Figurative sense of "burden weighing on the mind, heart, or soul" is first attested 1590s. Meaning "amount (of work, etc.) to be done by one person" is attested in compounds from 1939 (first was workload). Colloquial loads "lots, heaps" is attested from c. 1600. Phrase take a load off (one's) feet "sit down, relax" is from 1914, American English. Get a load of "take a look at" is American English colloquial, attested from 1929.