The adjective "mordant" comes from the Latin mordēre meaning "to bite", reflecting its original sense of something sharp, biting, or caustic, especially in speech or wit.  It was later used as a noun in dyeing meaning a substance that helps dye "bite" onto fabric.  It passed through Old French (mordant, (present participle of mordre, meaning "biting") before entering English.  It also gives us related words like "morsel" (a small bite) and "remorse" (to bite again).  Other related words, such as morbid, mortal, mortify, all stem from mordēre or related roots meaning to bite or harm. 

Examples of "mordant" used in a sentence: 
"Yet who could resist her mordant wit?" 
"His dialogue is sharp and witty, laced with mordant humor." 
"It is this mordant wit that I suspect has made him so successful." 
"Shot through with mordant wit, they make you nostalgic for the days before e-mail." 
"He was good company, with a mordant sense of humor and a strong sense of duty." 
"What's most distinctive about this mordant comedy of manners is the resolutely awkward cinematography." 
"I sent emails and hundreds of texts, trying to explain to our friends what was happening, masking my fear with mordant humor."