"that which is laid upon a person or beast, burden," c.1200, from Old English lad "way, course, carrying," from Proto-Germanic *laitho (cf. Old High German leita, German leite, Old Norse leið "way, course"); related to Old English lædan "to guide," from PIE *leit- "to go forth" (see lead (v.)). Sense shifted 13c. to supplant words based on lade, to which it is not etymologically connected; original association with "guide" is preserved in lodestone. Meaning "amount customarily loaded at one time" is from c.1300.
Figurative sense of "burden weighing on the mind, heart, or soul" is first attested 1590s. Meaning "amount of work" is from 1946. 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.
A departure from normal body content, as of water, salt, or heat. A positive load is a quantity in excess of the normal; a negative load is a deficit.