fence (n.) Look up fence at Dictionary.com
early 14c., shortening of defens (see defense). Spelling alternated between -c- and -s- in M.E. Sense of "enclosure" is first recorded 1510s. Fencible (early 14c.) means "capable of making a defense." Sense of "dealer in stolen goods" is thieves' slang, first attested c.1700, from notion of such transactions taking place under defense of secrecy. To be figuratively on the fence "uncommitted" is from 1828, from the notion of spectators at a fight.
fence (v.) Look up fence at Dictionary.com
"fight with swords," 1590s, first recorded in "Merry Wives of Windsor"; from the noun in this sense (1530s), see fence (n.). In spite of the re-enactment in 1285 of the Assize of Arms of 1181, fencing was regarded as unlawful in England. The keeping of fencing schools was forbidden in the City of London, "as fools who delight in mischief do learn to fence with buckler, and thereby are encouraged in their follies."