"to gain as a net sum, produce as clear profit," 1758, from net (adj.). Related: Netted; netting.
1650s, "an allowance by way of discount, deduction from a sum of money to be paid," from rebate (v.). By 1882 as "a repayment, money paid back."
Meaning "sanction, approval, tolerance" is from 1550s. Sense of "a sum allotted to meet expenses" is from c. 1400. In accounts, meaning "a sum placed to one's credit" is attested from 1520s. Mechanical meaning "permissible deviation from a standard" is from 1903. To make allowances is literally to add or deduct a sum from someone's account for some special circumstance; figurative use of the phrase is attested from 1670s.
c. 1400, "bring together in a sum or mass," from Latin aggregatus, past participle of aggregare "attach, join, include; collect, bring together," literally "bring together in a flock," from assimilated form of ad "to" (see ad-) + gregare "to collect into a flock, gather," from grex (genitive gregis) "a flock" (from PIE root *ger- "to gather"). Intransitive meaning "Come together in a sum or mass" is from 1855. Related: Aggregated; aggregating.