Signs for LEAN
The term "lean" and "lean on" as in "to rest against or on something for physical support" in ASL often require verb inflections and classifiers in verb predicates in several ways.
One example is a person leaning on the side of a car, using the dominant-handed classifier of the "upside down 2" handshape which represents a person standing. The dominant hand leans on the non-dominant classifier "3" handshape which represents one of the vehicles -- a car in this case.
Meaning: be in or move into a sloping position.
Then again, if you talk about "he leaned back in his chair", then you'd not use this sign above. Use a classifier predicate.
(of persons or animals) without much flesh or fat; not plump or fat, see THIN.
If talking about a meat, use fingerspelling (fs-LEAN).