1) As a verb
a. To treat someone with hatred or contempt
b. To consider someone beneath oneself
2) As a noun
a. A feeling of contempt
Disdain can be learnt easily associating it with two different words. Dis of disdain can be associated with this and dain of disdain can be associated with then. This and then form to make disdain and we can safely say with of sarcasm: this is what you then!