to unite or join so as to increase the number, quantity, size, or importance: to add two cups of sugar; to add a postscript to her letter; to add insult to injury.
2.
to find the sum of (often followed by up): Add this column of figures. Add up the grocery bills.
3.
to say or write further.
4.
to include (usually followed by in): Don't forget to add in the tip.
late 14c., from L. addere "add to, join" from ad- "to" + -dere comb. form meaning "to put, place," from dare "to give" (see date (1)). To add up "make sense" is from 1942. Add-on "additional component" is from 1941.