dependency (n.)

also dependant, late 14c., "relying for existence on;" early 15c. as "contingent, related to some condition;" from Old French dependant, present-participle adjective from dependre "to hang down," from Latin dependere "to hang from, hang down; be dependent on, be derived," from de "from, down" (see de-) + pendere "to hang, cause to hang; weigh" (from PIE root *(s)pen- "to draw, stretch, spin").

For spelling, see dependant (n.). In some cases the English word is directly from Latin dependentem (nominative dependens), present participle of dependere. From early 15c. in the literal sense of "hanging down, pendent." From 1640s as "subordinate, under the control of or needing aid from an extraneous source." Dependent variable in mathematics is recorded from 1852.

abstract noun suffix of quality or rank, from Latin -cia, -tia, from Greek -kia, -tia, from abstract ending -ia (see -ia) + stem ending -c- or -t-. The native correspondents are -ship, -hood.