thin and slim may mean the same thing but they are not synonyms. 'Slim' largely means being in a good shape not with big muscles or fats. The body is not. Slim definition is — of small diameter or thickness in proportion to the height or length of small diameter or See slim defined for English-language learners. Slim definition, slender, as in girth or form; slight in build or structure. See more. Chiefly British. to try to become more slender, especially by dieting. noun. 8. slim meaning, definition, what is slim: someone who is slim is attractively thin. From Longman Dictionary of Contemporary Englishslimslim1 /slɪm/ ○○○ S3. Synonyms for slim at hcaqualityreport.co.uk with free online thesaurus, antonyms, and definitions. In English 17c. also sometimes with a sense "sly, cunning, crafty. Definition of slim - (of a person or their build) gracefully thin; slender, (of something abstract, especially a chance or margin) very small, crafty.