"of the nature of, or caused by, a virus," 1944, see virus + -al (1). Sense of "become suddenly widely popular through internet sharing" is attested by 1999, originally in reference to marketing and based on the similarity of the effect to the spread of a computer virus. Related: Virally.
early 15c., aquitaille, "payment of debt or retribution;" see acquit + -al (2). Sense of "a release from debt or obligation" is from mid-15c.; that of "freeing from charge or offense" (by legal process) is from 1530s.
"setting of a price, valuation," by 1784, American English, from appraise + -al (2). The figurative sense of "act of appraising" (originally a term of literary criticism) is from 1817. Appraisement is earlier (1640s).