Green’s Dictionary of Slang

shameless, devoid of shyness.

1785, 1788, 1796		Grose Classical Dict. of the Vulgar Tongue n.p.: As being dipt in the river Styx was formerly supposed to render persons invulnerable, so it is said that a dipping in the river Shannon totally annihilates bashfulness; whence arises the saying of an impudent Irishman, that he has been dipt in the Shannon.
1811		Lex. Balatronicum [as cit. 1785].
1822		Royal Cornwall Gaz. 27 Apr. 3/1: I submit [...] that Misocant will appear to be a convicted fibber [...] and must have been thrice at the least dipped in the Shannon.
1823		Egan Grose’s Classical Dict. of the Vulgar Tongue.
1828		N. Devon Jrnl 5 June 1/5: He [...] must be the most impudent man ever dipped in the Shannon.
1838		Morn. Post (London) 1 Nov. 3/2: I am sure that even if nine times dipped on the Shannon, you will not venture to name the House of Commons.
1843		Kendal Mercury 1 Apr. 2/3: Paddy Ryan had evidently been ‘dipped in the Shannon, and kissed the Blarney stone’.
1861		Cork Examiner 12 July 3/6: He who has been dipped in the Shannon [...] is presumed to have obtained [...] the gift of that ‘civil courage’ which makes an Irishman at ease and unconstrained in all places.