The Collaborative International Dictionary of English v.0.48:

Insure \In*sure"\, v. i.
   To underwrite; to make insurance; as, a company insures at
   three per cent.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Insure \In"sure\, v. t. [imp. & p. p. Insured; p. pr. & vb. n.
   Insuring.] [OE. ensuren, prob. for assuren, by a change of
   prefix. See 1st In-, and Sure, and cf. Assure,
   Ensure.] [Written also ensure.]
   [1913 Webster]
   1. To make sure or secure; as, to insure safety to any one.
      [1913 Webster]

   2. Specifically, to secure against a loss by a contingent
      event, on certain stipulated conditions, or at a given
      rate or premium; to give or to take an insurance on or
      for; as, a merchant insures his ship or its cargo, or
      both, against the dangers of the sea; goods and buildings
      are insured against fire or water; persons are insured
      against sickness, accident, or death; and sometimes
      hazardous debts are insured.
      [1913 Webster]

WordNet (r) 3.0 (2006):

insure
    v 1: be careful or certain to do something; make certain of
         something; "He verified that the valves were closed"; "See
         that the curtains are closed"; "control the quality of the
         product" [syn: see, check, insure, see to it,
         ensure, control, ascertain, assure]
    2: make certain of; "This nest egg will ensure a nice retirement
       for us"; "Preparation will guarantee success!" [syn:
       guarantee, ensure, insure, assure, secure]
    3: protect by insurance; "The insurance won't cover this" [syn:
       cover, insure, underwrite]
    4: take out insurance for

Moby Thesaurus II by Grady Ward, 1.0:

75 Moby Thesaurus words for "insure":
   arm, armor, ascertain, assure, attest, back, be sponsor for, bless,
   bond, certify, champion, cinch, clear up, clinch, cloak,
   compass about, confirm, copyright, countersecure, cover, cushion,
   decide, defend, determine, dismiss all doubt, endorse, ensure,
   establish, fence, fend, find out, fix, get at, guarantee, guaranty,
   guard, harbor, haven, keep, keep from harm, make certain,
   make no doubt, make no mistake, make safe, make sure, make sure of,
   nail down, nestle, patent, police, protect, reassure, register,
   remove all doubt, ride shotgun for, safeguard, screen, secure,
   see that, see to it, set at rest, settle, shelter, shield, shroud,
   sign, sign for, sort out, sponsor, stand behind, stand up for,
   subscribe to, undersign, underwrite, warrant