. "Complain" and "plaintiff" are
distantly related; both can be traced back to "plangere," a Latin
word meaning "to strike, beat one's breast, or lament."
"Plaintiff" comes most immediately from Middle English
"plaintif," itself an Anglo-French borrowing tracing back to
"plaint," meaning "lamentation." (The English word
"plaintive" is also related.) Logically enough, "plaintiff"
applies to the one who does the complaining in a legal case.