2. To go on to say or to write more: Helen said goodbye and wanted to add that she had a pleasant visit with Darren and Yvonne.
3. To join one thing to another so as to increase the number, quantity, or the importance of something: Lynn decided to add a new wing to her house.
Francisco and Thelma placed an ad in the paper because they wanted to add a pool to their yard; however, after they decided to add all of the expenses, they found it much more feasible to invest in a wading pool instead.
Literally meaning is "at pleasure". This expression is popularly used as a noun phrase or modifier in English in the form "ad lib" to express absence of planning; for example, "His worst jokes were carefully planned ad libs."