a phrase, often a twitter hashtag (#sourlemonface), used to indicate that something is good, ill, crazy, awesome, an excellent showcase of skill, etc. typically used in the context of praising rap lyricism. the phrase is derived from the scrunching-up of one's face in response to a particularly sick line/set of lines, reminiscent of a reaction to a bitter taste, such as that of a lemon.
@randomtwitteruser1: YO i just heard that new @Eminem song. #sourlemonface
@randomtwitteruser2: RT @randomtwitteruser1: YO i just heard that new @Eminem song. #sourlemonface