The Collaborative International Dictionary of English v.0.48:

Sallow \Sal"low\, a. [Compar. Sallower; superl. Sallowest.]
   [AS. salu; akin to D. zaluw, OHG. salo, Icel. s["o]lr
   yellow.]
   Having a yellowish color; of a pale, sickly color, tinged
   with yellow; as, a sallow skin. --Shak.
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The Collaborative International Dictionary of English v.0.48:

Sallow \Sal"low\, v. t.
   To tinge with sallowness. [Poetic]
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         July breathes hot, sallows the crispy fields. --Lowell.
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The Collaborative International Dictionary of English v.0.48:

Sallow \Sal"low\ (s[a^]l"l[-o]), n. [OE. salwe, AS. sealh; akin
   to OHG. salaha, G. salweide, Icel. selja, L. salix, Ir. sail,
   saileach, Gael. seileach, W. helyg, Gr. "eli`kh.]
   1. The willow; willow twigs. [Poetic] --Tennyson.
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            And bend the pliant sallow to a shield. --Fawkes.
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            The sallow knows the basketmaker's thumb. --Emerson.
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   2. (Bot.) A name given to certain species of willow,
      especially those which do not have flexible shoots, as
      Salix caprea, Salix cinerea, etc.
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   Sallow thorn (Bot.), a European thorny shrub (Hippophae
      rhamnoides) much like an Elaeagnus. The yellow berries
      are sometimes used for making jelly, and the plant affords
      a yellow dye.
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WordNet (r) 3.0 (2006):

sallow
    adj 1: unhealthy looking [syn: sallow, sickly]
    n 1: any of several Old World shrubby broad-leaved willows
         having large catkins; some are important sources for
         tanbark and charcoal
    v 1: cause to become sallow; "The illness has sallowed her face"