The Collaborative International Dictionary of English v.0.48:

Fag \Fag\, v. i. [imp. & p. p. Fagged (f[a^]gd); p. pr. & vb.
   n. Fagging (f[a^]g"g[i^]ng).] [Cf. LG. fakk wearied, weary,
   vaak slumber, drowsiness, OFries. fai, equiv. to f[=a]ch
   devoted to death, OS. f[=e]gi, OHG. feigi, G. feig, feige,
   cowardly, Icel. feigr fated to die, AS. f[=ae]ge, Scot. faik,
   to fail, stop, lower the price; or perh. the same word as E.
   flag to droop.]
   1. To become weary; to tire.
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            Creighton withheld his force till the Italian began
            to fag.                               --G.
                                                  Mackenzie.
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   2. To labor to wearness; to work hard; to drudge.
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            Read, fag, and subdue this chapter.   --Coleridge.
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   3. To act as a fag, or perform menial services or drudgery,
      for another, as in some English schools.
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   To fag out, to become untwisted or frayed, as the end of a
      rope, or the edge of canvas.
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The Collaborative International Dictionary of English v.0.48:

fagging \fag"ging\ (f[a^]g"g[i^]ng), n.
   Laborious drudgery; esp., the acting as a drudge for another
   at an English school.
   [1913 Webster]