Something that is “half-baked” is something that is not fully cooked or fully developed. “Half-baked” can apply to a food (as the term originally was), or it can apply to an idea, a scheme, or a person.
 
‘Half-baked ship owner” is cited in 1827. “Half-baked” probably comes from England; it’s cited in the United States in the 1850s.
 
 
Merriam-Webster Online Dictionary
Main Entry: half–baked
Pronunciation: \-ˈbākt\
Function: adjective
Date: 1621
1 a: poorly developed or carried out “a half–baked idea” “half–baked research” b: lacking adequate planning or forethought “a half–baked scheme for getting rich” c: lacking in judgment, intelligence, or common sense
2: imperfectly baked : underdone
 
(Oxford English Dictionary)
half-baked, a.
1. lit. See HALF adv. and BAKED; hence, underdone, not thorough, not earnest; raw, crude, ill-digested; half-finished, incomplete, rude.
1621 SANDERSON 12 Serm. (1637) 330 Our profest Popelings, and halfe-baked Protestants.
a1628 PRESTON Serm. Bef. His Majestie (1630) 36 They are either done withoute heate, or but half-baked.
1824 SCOTT St. Ronan’s xxxi, He must scheme, forsooth, this half-baked Scotch cake!..this lump of oatmeal dough!
2. Deficient in intellect; silly, half-witted. dial.
1855 KINGSLEY Westw. Ho! iii. (D.), A sort of harmless lunatic, and, as they say in Devon, half-baked.
1893 Spectator 24 June 847 Nor could a special variety of intellectual feebleness be better described than by the epithet ‘half-baked’.
 
10 November 1827, The Times (London), pg. 3, col. 6:
Now, Sir, as to the policy of acceding to the Trinity proposition, leaving its national importance out of the argument, I should not despair of proving, even to a half-baked ship-owner, that as a matter of pounds, shillings, and even pence (even if the proposed rate were twenty times the paltry farthing per too), it would be much to his pecuniary advantage to promote the establishment of more lights.
 
Google Books
A dictionary of archaic and provincial words, obsolete phrases, proverbs, and ancient customs, from the fourteenth century
By James Orchard Halliwell-Phillipps
London: T. and W. Boone
1855
Pg. 429:
HALF-BAKED. Raw; inexperienced; half-silly. Var. dial.
   
1 July 1856, Wisconsin Mirror (Kilbourn City, WI), pg. 2, col. 7:
Fremont was then declared unanimously nominated, with cheers after cheers, that were taken up by the crowd in the streets, and echoed so heartily that it almost shook the city of brocks, and will be likely to shake to pieces the half-baked dougfaces (sic) of Pennsylvania and the whole north!
 
17 July 1857, Newport (RI) Daily News, pg. 2, col. 4:
There is, consequently, a desperate “roasting out” of our denizens for the country—the multitudinous steamers and rail-cars, that are incessantly radiating from this great centre, being literally crammed and jammed with half-baked passengers, and all manner of trunks, bags, boxes, and other terms of luggage.
 
19 June 1858, Daily Wisconsin Patriot (Madison, WI), pg. 2, col. 1:
...I shipped 128 lbs. of freight on board of the cars for Milwaukee, determined to be in this city in time to meet my friend Sharpsteal on his return from the great Mecca of sore heads, and half-baked politicians, to condole (console?—ed.) with him in his late bereavement.