The word 'please' entered English through the Norman Conquest's lasting reorientation of the language toward French and Latin sources. Its immediate ancestor is Old French plaisir (to please, pleasure), which derived from Latin placēre, meaning 'to be agreeable' or 'to give pleasure'. This Latin verb was itself a descendant of the PIE root *pleh₂k-, which carried the sense of being flat, broad, or spread out.
The connection between flatness and pleasantness is ancient and cross-cultural. A flat, smooth surface offers no resistance, no friction, no obstruction. In the PIE conceptual system, this physical quality translated readily into a metaphorical one: what is flat is what agrees with you, what accommodates rather than confronts. The same root produced Latin placidus (calm, gentle — whence English 'placid'), placare (to soothe, appease — whence 'placate'), and complacēre (to be very pleased — whence 'complacent
Through a related but divergent pathway, the same PIE root yielded the word 'place' and its Spanish and English descendant 'plaza'. A plaza is literally a broad, flat, open space — the PIE root's physical sense maintained in its most literal form. The irony is that 'plaza' (a flat, open public space) and 'please' (to give satisfaction) look utterly different yet share the same ancestry.
In Middle English, the verb appears as plesen, plesen, or plesir, borrowed from Old French in the 14th century as part of the vast influx of French vocabulary that followed 1066. The word quickly established itself in the core vocabulary, displacing or supplementing Old English expressions like licigan (to please, to be agreeable to — whence modern 'like'). 'Like', in its sense of 'to be pleasing to', is the native Germanic equivalent of 'please': both mean something very similar but come from entirely different branches of the Indo-European family.
The development of 'please' as a polite adverb or formula — used in requests to signal deference or courtesy — is a secondary development. The phrase 'if it pleases you' or 'if you please' was shortened in conversational use to simply 'please', functioning as a politeness marker detached from its verbal meaning. This type of grammaticalisation — where a full phrase shrinks into a single pragmatic particle — is common in the history of politeness expressions across many languages.
The noun 'pleasure' entered English slightly earlier than 'please' as a verb, coming directly from Old French plaisir used as a noun. In early use, 'pleasure' carried a wider sense of will or choice ('at the king's pleasure') as well as the sense of sensory or emotional enjoyment. This broader sense reflects Latin placitum (what has been decided, what has been agreed to), the past participle of placēre, used in legal and official contexts.
The antonym 'displease' formed naturally by prefixation, and 'pleasant', 'pleasantry', and 'pleasurable' extended the family further. The entire cluster — please, pleasure, pleasant, placid, placate, complacent, implacable, complaisant — represents one of the most coherent semantic families in English, all radiating outward from the Latin root and its PIE ancestor.
In modern usage, 'please' is one of the most culturally significant words in the language: its use or omission is a primary marker of politeness and social register. Children are taught to say 'please' as one of the first social lessons, and its absence in requests is typically registered as rudeness. That such a socially charged word traces back to a metaphor about smooth, flat surfaces is one of the small marvels of historical linguistics.