Behaving in a calm, polite way — not rude, not noisy, not rough. It is used most about children, students, and animals: a well-behaved class, a well-behaved dog. Used about an adult it usually sounds a little funny or ironic, because we expect adults to behave anyway: 'the guests were surprisingly well-behaved' hints that you thought they might not be. Two things worth noticing. First, it is mostly praise coming from someone with authority looking down — a teacher, a parent, a host. That is why it can sting when said about a grown person; it puts them in the position of a child. Second, English also uses it in a slightly technical way for things that are not people at all: scientists and programmers speak of a 'well-behaved function' or 'well-behaved code', meaning it does what it is supposed to do without surprises. Same idea, no politeness involved. Keep the hyphen before a noun ('a well-behaved boy'); after the verb it is often written open ('the children were well behaved').