listless

from The American Heritage® Dictionary of the English Language, 5th Edition.

• adjective Lacking energy or disinclined to exert effort; lethargic.
• adjective Characterized by a lack of energy or interest.

from The Century Dictionary.

• Indifferent to or taking no interest in what happens about one; languid and unheeding: as, a listless hearer or spectator.
• Marked by languid inactivity; manifesting relaxed attention; inanimate: as, a listless attitude.
• Synonyms Listless, Careless, Supine, Indolent. The listless and the careless do not care or desire; the supine and the indolent do not care enough to conquer their shrinking from activity or work. The words may all indicate a temporary state or a permanent element of character; indolent generally indicates the latter. (See idle.) Careless is not caring; supine is literally lying flat on one's back, not rousing one's self at all, ignobly indifferent; listless, indifferent and languid. Listless does not necessarily imply blame.

from the GNU version of the Collaborative International Dictionary of English.

• adjective Having no desire or inclination; indifferent; heedless; spiritless.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• adjective Lacking energy, enthusiasm, or liveliness.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• adjective marked by low spirits; showing no enthusiasm
• adjective lacking zest or vivacity