(A.) A surgeon's mate in the navy. Here lob is the Welsh llob, a dolt, and loblolly boy is a dolt not yet out of his spoon-meat or baby-pap.
“Loblolly-boy is a person on board a man-of-war who attends the surgeon and his mates, but knows as much about the business of seaman as the author of this poem.” —The Patent (1776).
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894
More on Loblolly Boy from Fact Monster: