This word has greatly changed its original meaning. The
Romans used it for a thanksgiving after a signal victory (Livy, iii. 63). (“His rebus gestis, supplicatio a senatu decreta est” [Caesar: Bell. Gall.,
ii.].) The word means the act of folding the knees (sub-plico). We now use the word for begging or entreating something.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894