Green’s Dictionary of Slang

a fool, a simpleton; also attrib.

1833		[T. Wontner] Old Bailey Experience 398: In every small band [...] of young thieves, there will always be found one or two sillikins, as they denominate those whom they can persuade to be foremost in any undertaking, by taunts of cowardice and threats of dissolving partnership.
1859		Mons. Beacon 24 Dec. 4/6: Poor sillikin! he knows nothing of the secret clause in the treaty with Mr. Chopperbloc.
1865		Leaves from Diary of Celebrated Burglar 37/2: ‘Keep on your “tog,” you b—y “silliken,”’ answered Jemmy, in a loud tone.
1874		Paisley Herald 28 Mar. 5/4: I think the Yankees deserve less blame [...] than the sillikin clericals who are playing second fiddle to them.
1876		Era (London) 10 Sept. 5/1: Mr W.B. Fair, in the garb of a country numpkin, sang ‘I’m such a sillikin’ in very humourous style.
1882		Dundee Eve. Teleg. 15 Feb. 4/3: ‘As fer the gerls [...] I think they must be sillikins’.
1885		Bulletin (Sydney) 17 Jan. 22/4: Any number of sweet souled sillikins, from twelve up to twelve millions, can be put through in half an hour, at most, and all that’s required is one service, one officiating clergyman, one fee (clubbed up on the cooperative plan), one organ, one wedding breakfast, one monster bridecake, one comprehensive toast embracing all the brides and another including all the bridegrooms.
1893		Belfast News Letter 26 Dec. 6/5: She laughed, and decared I was a sillikin.
1925		Nottingham Eve. Post 9n Sept. 3/1: ‘Ting-a-ling, ting, ting, tay!’ Your chum Sillikins.