1. (v. t.) To make strong or stronger; to add strength to; as, to strengthen a limb, a bridge, an army; to strengthen an obligation; to strengthen authority.

2. (v. t.) To animate; to encourage; to fix in resolution.

3. (v. i.) To grow strong or stronger.

accelerate accrue accumulate add to advance affirm aggravate animate anneal appreciate attest augment authenticate back back up balloon be tough bear out beef up bloat blow up bolster boom boost brace brace up breed broaden buttress case harden certify cheer chirk up circumstantiate complicate concentrate condense confirm consolidate corroborate crescendo deepen develop document double embolden encourage endure energize enhance exacerbate exaggerate fortify gain gain strength gelt get ahead gird go up grow hang tough harden hearten heat up heighten hop up hot up increase inspirit intensify invigorate jazz up key up magnify make complex mount multiply nerve nourish nutrify prepare probate proliferate prop prove ramify ratify ready recruit redouble refresh reinforce reinvigorate rejuvenate renew restrengthen rigidify rise run up screw up sharpen shoot up shore up sinew snowball soup up spread steel step up stiffen substantiate supplement support sustain swell temper tense tighten tone up toughen trice up triple undergird uphold validate verify vitalize warrant wax whet widen