Abbreviation for Latin "ad libitum" indicating "at enjoyment" and "at a person's satisfaction, around one desires, fully extent of one's desires." Sometimes seen on a prescription or doctor's order. For instance, during an instantly fast once the patient is certainly not to eat any meals but could have water, the doctor's order might review: "liquid ad lib" (liquid as desired).
1811, shortened from Latin ad libitum "at a person's satisfaction, whenever one wants" (c.1600), from advertisement "to" (see ad-) + libitum, accusative of libere "to kindly" (see libido). Initially recorded as you term 1919 (v.), 1925 (letter.).
whenever needed or wanted; quick kind of your message ad libitum
without advance planning