Lamar is an Americanized version of the French surname Lamare, indicating an ancestor who lived by a pool or pond. It became a regularly used name in the Southern part of the USA during the 19th century because it was the surname of some prominent political figures, including Mirabeau Lamar of Texas and
Lucius Lamar of Mississippi:
http://www.lsjunction.com/people/lamar.htm
http://bioguide.congress.gov/scripts/biodisplay.pl?index=L000030