alibi
A claim of having been elsewhere, and the looser use for any excuse.
An alibi is a claim or proof that someone was elsewhere when an act occurred: His alibi was a hotel bill from Denver. A common error is to use alibi to mean an ordinary excuse, as in Her alibi for the late report was traffic. For a reason offered to avoid blame, use excuse or explanation: Her excuse for the late report was traffic.
Examples
UseHer alibi was a time-stamped receipt from another city. (proof that she was elsewhere)
UseHe offered an excuse for missing the deadline. (a reason given to avoid blame)
AvoidHe offered an alibi for missing the deadline. (if you mean an excuse)
Dictionaries now also list alibi meaning “excuse.” However, many people still consider this use an error, and they judge the speaker who uses alibi this way.