To eat a lemon is to experience the bitter, sour and unpleasant reality of rejection. This phrase is most commonly used to describe romantic rejection and while one can lament the misfortune of having to eat a lemon, one can also coo in the sadistic satisfaction of treating a poor soul to a lemon as in “我請𠈄食檸檬”
Lemon eating is not exclusively limited to failed romantic pursuits as it can also be used to describe an unexpectedly dismal reaction. For example, your boss, in an attempt to raise company morale announces a team building karaoke retreat. Unfortunately, upon hearing the lacklustre groan, he immediately recognises that he’s just eaten a lemon