From the verb to stick (to fasten, make firm, fix).
Live stock. The fixed capital of a farm. Stock in trade. The fixed capital.
The village stocks, in which the feet are stuck or fastened. A gun stock, in which the gun is stuck or made fast.
It is on the stocks. It is in hand, but not yet finished. The stocks is the frame in which a ship is placed while building, and so long as it is in hand it is said to be or to lie on the stocks.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894