The Collaborative International Dictionary of English v.0.48:

Tack \Tack\, v. t. [imp. & p. p. Tacked; p. pr. & vb. n.
   Tacking.] [Cf. OD. tacken to touch, take, seize, fix, akin
   to E. take. See Tack a small nail.]
   1. To fasten or attach. "In hopes of getting some commendam
      tacked to their sees." --Swift.
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            And tacks the center to the sphere.   --Herbert.
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   2. Especially, to attach or secure in a slight or hasty
      manner, as by stitching or nailing; as, to tack together
      the sheets of a book; to tack one piece of cloth to
      another; to tack on a board or shingle; to tack one piece
      of metal to another by drops of solder.
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   3. In parliamentary usage, to add (a supplement) to a bill;
      to append; -- often with on or to; as, to tack on a
      non-germane appropriation to a bill. --Macaulay.
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   4. (Naut.) To change the direction of (a vessel) when sailing
      closehauled, by putting the helm alee and shifting the
      tacks and sails so that she will proceed to windward
      nearly at right angles to her former course.
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   Note: In tacking, a vessel is brought to point at first
         directly to windward, and then so that the wind will
         blow against the other side.
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The Collaborative International Dictionary of English v.0.48:

Tacking \Tack"ing\, n. (Law)
   A union of securities given at different times, all of which
   must be redeemed before an intermediate purchaser can
   interpose his claim. --Bouvier.
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   Note: The doctrine of tacking is not recognized in American
         law. --Kent.
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WordNet (r) 3.0 (2006):

tacking
    n 1: a loose temporary sewing stitch to hold layers of fabric
         together [syn: baste, basting, basting stitch,
         tacking]
    2: (nautical) the act of changing tack [syn: tack, tacking]

Bouvier's Law Dictionary, Revised 6th Ed (1856):

TACKING, Eng. law. The union of securities given at different times, so as
to prevent any intermediate purchasers claiming title to redeem, or
otherwise discharge one lien, which is prior, without redeeming or
discharging other liens also, which are subsequent to his own title. Jer.
Eq. Jur. B. 1, c. 2, Sec. 1, p. 188 to 191; 1 Story, Eq. Jur. Sec. 412.
     2. It is an established doctrine in the English chancery that a bona
fide purchaser and without any notice of a defect in his title at the time
of the purchase, may lawfully buy any statute, mortgage, or encumbrance, and
if he can defend by those at law, his adversary shall have no help in equity
to set those encumbrances aside, for equity will not disarm such a
purchaser. And as mortgagees are considered in equity as purchasers pro
tanto, the same doctrine has extended to them, and a mortgagee who has
advanced his money without notice of any prior encumbrance, may, by getting
an assignment of a statute, judgment, or recognizance, protect himself from
any encumbrance subsequent to such statute, judgment or recognizance, though
prior to his mortgage; that is, he will be allowed to tack or unite his
mortgage to such old security, and will by that means be entitled to recover
all moneys for which such security was given, together with the money due on
his mortgage, before the prior. mortgagees are entitled to recover anything.
2 Fonb. Eq. 306; 2 Cruise, t. 15, c. 5, s. 27; Powell on Morg. Index, h.t.;
1 Vern. 188; 8 Com. Dig. 953; Madd. Ch. Index, h.t.
     3. This doctrine is inconsistent with the laws of the several states,
which require the recording of mortgages. Caines' Cas. Er. 112; 1 Hop. C. R.
231; 3 Pick. 50; 2 Pick. 517.
     4. The doctrine of tacking seems to have been acknowledged in the civil
law, Code, 8, 27, 1; but see Dig. 13, 7, 8; and see 7 Toull. 110. But this
tacking could not take place to the injury of intermediate encumbrancers.
Story on Eq. Sec. 1010, and the authorities cited in the note.