According to the Lewis concept, an acid is a chemical species that can accept an electron pair. Boron in $(CH_3)_3B$ (trimethylborane) has only six electrons in its valence shell after forming three single bonds. Being an electron-deficient species, it can accept an electron pair to complete its octet, thus acting as a Lewis acid. In contrast, oxygen in $(CH_3)_2O$, phosphorus in $(CH_3)_3P$, and nitrogen in $(CH_3)_3N$ have lone pairs of electrons that they can donate, meaning they act as Lewis bases.