'tranquilize' originates from English formation of the adjective 'tranquil' plus the verb-forming suffix '-ize', ultimately from Latin 'tranquillus', where 'tranquillus' meant 'calm, quiet'.

'tranquilize' was formed in English (19th century) from 'tranquil' + '-ize'; 'tranquil' came into English via Old French from Latin 'tranquillus', and the verb-forming suffix derives from Greek/French/Latin suffixal formations that entered English.

Initially it meant 'to make calm' (in a general sense); over time the meaning has largely remained but has come to be commonly used with the additional sense 'to sedate (with drugs)'.