the quality of mercy is not strained

A line from the play The Merchant of Venice, by William Shakespeare. Strained means “constrained,” or “forced”; the speaker is telling Shylock that mercy must be freely given, and is inviting him to show mercy to the title character. Mercy is something that has to be freely given; no one can force someone else to be merciful. (“Strained” is an old form of “constrained,” meaning “forced.”) From The Merchant of Venice by William Shakespeare.