The Latin verb placere, meaning “be acceptable” or “be liked,” is the source of a number of English words pertaining to agreeability. This post lists and defines these terms.
The verb please, meaning “be agreeable,” is from plaisir, the Old French intermediary of placere, which is also the origin of pleasure, meaning “give pleasure to” and, as a noun, “enjoyment” or “something enjoyed.” (The synonym pleasance is rare.) Please, as an adverb, also precedes or follows a request (“Please close the door”) or an affirmation (“Yes, please”) to make it more courteous but can also express derision (“Oh, please”). The antonyms of the verb and noun, respectively, are displease and displeasure. Something that pleases is pleasing, and someone who pleases is a pleaser.
Placebo was adopted directly from a Latin term meaning “I will please.” The word refers to a substance given to a patient for mental rather than physical relief (because the patient is deceived into thinking that the placebo will have an effect on an imagined or exaggerated condition). It also applies to a similarly innocuous substance given to members of a control group in an experiment to help evaluate the effectiveness of a drug taken by test subjects; if the drug is determined to be more effective than the placebo, it is efficacious.
Another close relative is the adjective pleasant, meaning “agreeable,” the antonym of which is unpleasant. A pleasantry is a polite remark made during a social occasion, or a humorous or playful one. Meanwhile, plea is also descended from placere, by way of placitum, meaning “decree” or “opinion”; the evolution of the sense is from “something agreed on because it pleases” to “something decided.” The verb form, plead, originally applied exclusively to making a plea in court but was later extended to apply to an urgent request; pleading developed as a noun from the first sense, describing the act of arguing a case.
Complacence originally meant “pleasure” but came to pertain to a lack of concern or to self-satisfaction; the variation complacency has the negative connotation of clueless complacence. The homophone complaisance is a direct borrowing from Middle French meaning “desire to please.” The adjectival forms are complacent and complaisant.
Placid, from placere by way of placidus, meaning “peaceful,” retains the Latin meaning. (The noun form is placidity.)
The Latin verb placare, meaning “appease” or “assuage,” is related; it is the source of placate, which retains those meanings. The adjectival form is placatory, and the act is called placation. Placable is an alternative adjective, though it is rare; the antonym, implacable, is more common.