WordWeb Online

1. Lacking ardour, vigour or energy
   "a spiritless reply to criticism"
    
2. Evidencing little spirit or courage; overly submissive or compliant
   "a fine fiery blast against spiritless conformity";
   - meek

See also: animation, apathetic, bloodless, brio, dispirited, dull, heartless [archaic], hebetudinous [rare], invigoration, listless, meh [informal], spiritedness, submissive, thin, unenthused, unenthusiastic, vivification

Antonym: spirited