retain (v.)

late 14c., "continue keeping of, keep possession of, keep attached to one's person;"  early 15c., "hold back, restrain" (a sense now obsolete); from Old French retenir "keep, retain; take into feudal service; hold back; remember" (12c.), from Latin retinere "hold back, keep back, detain, restrain," from re- "back" (see re-) + tenere "to hold" (from PIE root *ten- "to stretch").

The meaning "to engage to keep (another) attached to one's person, keep in service" is from mid-15c.; specifically of lawyers from 1540s. Meaning "keep in the mind, preserve knowledge or an idea of" is from c. 1500. Related: Retained; retaining.

Definitions of retain

retain (v.)
hold back within;
I retain this drug for a long time
This soil retains water
the dam retains the water
retain (v.)
allow to remain in a place or position or maintain a property or feature;
this garment retains its shape even after many washings
She retains a lawyer
She retained her composure
Synonyms: continue / keep / keep on
retain (v.)
secure and keep for possible future use or application;
The landlord retained the security deposit
Synonyms: hold / keep back / hold back
retain (v.)
keep in one's mind;
I cannot retain so much information