1 join (one thing to another) as an increase or supplement (add your efforts to mine; add insult to injury).
2 put together (two or more numbers) to find a number denoting their combined value.
3 say in addition (added a remark; added that I was wrong).
Phrases and idioms:
add in include. add-on something added to an existing object or quantity. add to increase; be a further item among (this adds to our difficulties). add up
1 find the total of.
2 (foll. by to) amount to; constitute (adds up to a disaster).
3 colloq. make sense; be understandable.
Derivatives:
added adj.
Etymology: ME f. L addere (as AD-, dare put)
Useful english dictionary. 2012.