Yours, Etc.: Origins and Uses of 8 Sign-Offs
"U.S. soldier," 1864, American English, said to have been in oral use from 1854, or from the Mexican-American War (1847), it is perhaps from resemblance of big buttons on old uniforms to a sort of biscuit of that name (1680s), but there are various other conjectures.
An infantry soldier; grunt, paddlefoot
[1867+; origin unknown; perhaps fr a resemblance between the buttons of the infantry uniform and doughboys, ''suet dumplings boiled in seawater,'' a term fr the British merchant marine]