Why do inflammable and infamous have somewhat counterintuitive meanings?
“Infamous” is the adjective form of “infamy,” and is not related to “famous” as far as I know. Not sure about inflammable.
in- as a prefix has several meanings. The most common is “not,” however other meanings include “inward,” “intensive action,” and “to cause.” Thus, “infamous” derives for “not” (originally, it meant “not famous”). “Inflammable” derives from “to intensify.” An inflammable liquid causes flames to intensify.
As shown below, “famous” and “infamous” both ultimately derive from Latin “fama” via slightly different routes.
http://www.m-w.com/cgi-bin/dictionary?book=Dictionary&va=infamous
http://www.m-w.com/cgi-bin/dictionary?book=Dictionary&va=infamy