British Regulars were the foot soldiers of the British Army. You'll also hear them referred to derisively as "Redcoats" because of the red coats they wear as part of their uniforms (I'll bet you didn't see that coming) or "Lobsterbacks" because of their huge, deadly pincers. Or still because of the red coat thing, I can't remember which.
Regulars in the colonies were notoriously underpaid, and many looked for work outside of their army duties to make ends meet. Of course, their room and board was paid by the government, meaning they could charge lower-than-average rates for their work. Since unemployment was high in Boston before the Revolution, you can imagine this made the Redcoats rather unpopular among people they were taking work from. That, and their huge, deadly pincers.