The Collaborative International Dictionary of English v.0.48:

Strengthener \Strength"en*er\, n.
   One who, or that which, gives or adds strength. --Sir W.
   Temple.
   [1913 Webster]

WordNet (r) 3.0 (2006):

strengthener
    n 1: a device designed to provide additional strength; "the
         cardboard backing was just a strengthener"; "he used gummed
         reinforcements to hold the page in his notebook" [syn:
         strengthener, reinforcement]

Moby Thesaurus II by Grady Ward, 1.0:

54 Moby Thesaurus words for "strengthener":
   advocate, alpenstock, arm, athletic supporter, back, backbone,
   backing, bandeau, bearer, bra, brace, bracer, bracket, brassiere,
   buttress, cane, carrier, cervix, corset, crook, crutch,
   foundation garment, fulcrum, girdle, guy, guywire, jock, jockstrap,
   mainstay, maintainer, mast, neck, prop, reinforce, reinforcement,
   reinforcer, rest, resting place, rigging, shoulder, shroud, spine,
   sprit, staff, standing rigging, stave, stay, stick, stiffener,
   support, supporter, sustainer, upholder, walking stick