Jamieson, Fausset and Brown on Luke 18:13

13. standing afar off--as unworthy to draw near; but that was the way to get near (Psa 34:18; Is 57:15).

would not lift up--blushing and ashamed to do so (Ezr 9:6).

smote, &c.--kept smiting; for anguish (Lu 23:48), and self-reproach (Jr 31:19).

be merciful--"be propitiated," a very unusual word in such a sense, only once else used in the New Testament, in the sense of "making reconciliation" by sacrifice (He 2:17). There may therefore, be some allusion to this here, though not likely.

a sinner--literally, "the sinner"; that is, "If ever there was one, I am he."