adj. Having no obvious cause; fortuitous or inexplicable
adj. groundless or unreasonable
the GNU version of the Collaborative International Dictionary of English
adj. 1. Self-originating; uncreated.
adj. Without just or sufficient reason; groundless.
ad. Without cause or reason.
The Century Dictionary and Cyclopedia
Having no cause or producing agent; self-originated; uncreated.
Without just ground, reason, or motive: as, causeless hatred; causeless fear.
Without cause.
WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.
adj. having no cause or apparent cause
adj. having no justifying cause or reason
Word Usage
"If any human acts may loosely be called causeless, they are the minor acts of a healthy man; whistling as he walks; slashing the grass with a stick; kicking his heels or rubbing his hands."