The Collaborative International Dictionary of English v.0.48:

Tug \Tug\, v. i.
   1. To pull with great effort; to strain in labor; as, to tug
      at the oar; to tug against the stream.
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            He tugged, he shook, till down they came. --Milton.
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   2. To labor; to strive; to struggle.
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            England now is left
            To tug and scamble and to part by the teeth
            The unowed interest of proud-swelling state. --Shak.
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The Collaborative International Dictionary of English v.0.48:

Tug \Tug\, n.
   1. A pull with the utmost effort, as in the athletic contest
      called tug of war; a supreme effort.
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            At the tug he falls,
            Vast ruins come along, rent from the smoking walls.
                                                  --Dryden.
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   2. A sort of vehicle, used for conveying timber and heavy
      articles. [Prov. Eng.] --Halliwell.
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   3. (Naut.) A small, powerful steamboat used to tow vessels;
      -- called also steam tug, tugboat, and towboat.
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   4. A trace, or drawing strap, of a harness.
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   5. (Mining.) An iron hook of a hoisting tub, to which a
      tackle is affixed.
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   Tug iron, an iron hook or button to which a tug or trace
      may be attached, as on the shaft of a wagon.
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The Collaborative International Dictionary of English v.0.48:

Tug \Tug\, v. t. [imp. & p. p. Tugged; p. pr. & vb. n.
   Tugging.] [OE. toggen; akin to OD. tocken to entice, G.
   zucken to jerk, draw, Icel. toga to draw, AS. t['e]on, p. p.
   togen, to draw, G. ziehen, OHG. ziohan, Goth. tiuhan, L.
   ducere to lead, draw. Cf. Duke, Team, Tie, v. t.,
   Touch, Tow, v. t., Tuck to press in, Toy a
   plaything.]
   1. To pull or draw with great effort; to draw along with
      continued exertion; to haul along; to tow; as, to tug a
      loaded cart; to tug a ship into port.
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            There sweat, there strain, tug the laborious oar.
                                                  --Roscommon.
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   2. To pull; to pluck. [Obs.]
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            To ease the pain,
            His tugged cars suffered with a strain. --Hudibras.
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WordNet (r) 3.0 (2006):

tug
    n 1: a sudden abrupt pull [syn: tug, jerk]
    2: a powerful small boat designed to pull or push larger ships
       [syn: tugboat, tug, towboat, tower]
    v 1: pull hard; "The prisoner tugged at the chains"; "This movie
         tugs at the heart strings"
    2: strive and make an effort to reach a goal; "She tugged for
       years to make a decent living"; "We have to push a little to
       make the deadline!"; "She is driving away at her doctoral
       thesis" [syn: tug, labor, labour, push, drive]
    3: tow (a vessel) with a tug; "The tugboat tugged the freighter
       into the harbor"
    4: carry with difficulty; "You'll have to lug this suitcase"
       [syn: lug, tote, tug]
    5: move by pulling hard; "The horse finally tugged the cart out
       of the mud"
    6: pull or strain hard at; "Each oar was tugged by several men"
    7: struggle in opposition; "She tugged and wrestled with her
       conflicts"