Does not know "come here" from "sic them" is an idiom to denote complete ignorance. The person or thing that is being discussed is being compared to a dog that does confuses the command to "come here" with the command to "sic them" (that is to attack). When spoken with a thick Texas accent, the phrase rolls of the tongue as "don't know comer from sikum." It has also been shorted to "Here from sic 'im".[1]
References
- ↑ Safire, William. "ON LANGUAGE; Sic 'Em!", New York Times, June 13, 1993. Retrieved on February 6, 2015.