(verb) strengthen, augment
Synonyms: add fuel to fire, add to, back up, beef up, bolster, boost, build up, buttress, carry, emphasize, energize, enlarge, fortify, harden, heat up, hype, increase, lend a hand, multiply, pick up, pillar, prop, prop up, punch up, shore up, soup up, stand up for, stiffen, stress, stroke, supplement, support, sustain, toughen, underline
Antonyms: subtract, take away, undermine, weaken