means A spoken talk delivered to an audience to teach or inform them about a subject, or, less formally, a long scolding aimed at correcting someone's behavior.

from From the Latin 'lectura,' meaning 'a reading,' rooted in 'legere,' 'to read' — the same verb that gives us 'legible' and 'lecture's' bookish cousin 'lectern.' In medieval universities, a lecture was literally that: a master reading aloud from a precious, hand-copied manuscript while students scribbled, since printed books didn't yet exist to do the reading for them. The 'telling someone off' sense came later, the idea of being read a stern improving text.