The connotation of a word is its suggested or emotional meaning, as compared to its literal meaning. Connotations can be negative or positive, and are often subjective.
'Thin' is more or less neutral, 'slim' may have a positive connotation, 'skinny' a negative one.
In the classroom
Teachers often teach negative and positive connotations along with literal meaning. It is important to raise learners' awareness of the connotations of a word if they are to use them appropriately, although it is a complex and changeable area.