apt, apt.

Apt or apt means likely; inclined or disposed--
He is apt to lose his temper when things don't go as planned.
Things are apt to get hectic when we have a rush order.
He is an apt pupil, and learns quickly.

Apt or apt also means fitting; suitable to the purpose or occasion--
A few apt remarks will get the audience's attention.
It was an apt observation, if somewhat crude.

Apt. or apt. is an abbreviation of "apartment", meaning  a room or rooms for rent, for use as a dwelling--
The sign says "apt. for rent"; let's call and ask about it.
Look in the real estate ads under "apt.".

Now that you know that, you can say--
"He is apt to make spelling mistakes; tell him to simply write "apt." on the sign."

Note--not to be confused with "appt.", an abbreviation of "appointment", meaning a meeting agreed upon in advance--
His notebook sats "appt. 10 am", but it doesn't say where, or with whom.