straggler

from The Century Dictionary.

• noun One who straggles or strays away, as from his fellows or from the direct or proper course; one who lags behind or becomes separated in any way from his companions, as from a body of troops on the march.
• noun Specifically, in ornithology, a stray, or strayed bird, out of its usual range, or off its regular migration.
• noun One who roams or wanders about at random, or without settled direction or object; a wanderer; a vagabond; especially, a wandering, shiftless fellow; a tramp.
• noun Something that shoots beyond the rest or too far; an exuberant growth.
• noun Something that stands apart from others; a solitary or isolated individual.

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

• noun One who straggles, or departs from the direct or proper course, or from the company to which he belongs; one who falls behind the rest; one who rambles without any settled direction.
• noun A roving vagabond.
• noun Something that shoots, or spreads out, beyond the rest, or too far; an exuberant growth.
• noun Something that stands alone or by itself.

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

• noun One who straggles, or departs from the direct or proper course, or from the company to which he belongs.
• noun One who falls behind the rest.
• noun One who rambles without any settled direction.
• noun A roving vagabond.
• noun Something that shoots, or spreads out, beyond the rest, or too far; an exuberant growth.
• noun Something that stands alone or by itself.

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

• noun someone who strays or falls behind