Are 'bony', 'thin', 'lanky', 'slim' similar? Are they difference in tone, formality and politeness?
1. Slim, trim, slender and lean have a positive/approving nuance.
2. Thin can have both approving and disapproving nuance depending on context.
3. Skinny and bony have a disapproving nuance.
Of course, to a person who has just followed a very strict diet, it can only be a compliment to tell her: oh, you look skinny!
Lanky: A colloquial expression about a lanky person-- "he's all knees and elbows."