fenceyoudaoicibaDictYouDict[fence 词源字典]
fence: [14] Fence is short for defence, and indeed until the 16th century meant ‘defence’ (‘Yet, for [that is, despite] the fence that he could make, she struck it from his hand’, Felon Sowe Rokeby 1500). Of its present-day meanings, ‘enclosing structure’ (originally a ‘defence’ against intruders) and ‘sword-fighting’ (originally the use of a sword for ‘self-defence’, now used only as a verb) developed in the 16th century.

The sense ‘dealer in stolen property’ came along in the 17th century; it arose from the notion that such transactions are carried out under the cover, or ‘defence’, of secrecy. Similarly, fend [13] and fender [15] came by loss of the initial syllable from defend.
=> defence, fend, fender[fence etymology, fence origin, 英语词源]
fence (n.)youdaoicibaDictYouDict
early 14c., "action of defending, resistance; means of protection, fortification," shortening of defens (see defense). The same pattern also yielded fend, fender; and obsolete fensive "defensive" (late 16c.). Spelling alternated between -c- and -s- in Middle English. Sense of "enclosure" is first recorded mid-15c. on notion of "that which serves as 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, perhaps from the notion of spectators at a fight, or a simple literal image: "A man sitting on the top of a fence, can jump down on either side with equal facility." [Bartlett, "Dictionary of Americanisms," 1848].
fence (v.)youdaoicibaDictYouDict
early 15c., "defend" (oneself); mid-15c. as "protect with a hedge or fence;" from fence (n.). From 1590s as "fight with swords," from the noun in this sense (1530s); see fencing. From 1610s as "knowingly buy or sell stolen goods." Related: Fenced.