tenet (n.)
"principle, opinion, or dogma maintained as true by a person, sect, school, etc.," properly "a thing held (to be true)," early 15c., from Latin tenet "he holds," third person singular present indicative of tenere "to hold, grasp, keep, have possession, maintain," also "reach, gain, acquire, obtain; hold back, repress, restrain;" figuratively "hold in mind, take in, understand."

The Latin word is from PIE root *ten- "to stretch" (cognates: Sanskrit tantram "loom," tanoti "stretches, lasts;" Persian tar "string;" Lithuanian tankus "compact," i.e. "tightened;" Greek teinein "to stretch," tasis "a stretching, tension," tenos "sinew," tetanos "stiff, rigid," tonos "string," hence "sound, pitch;" Latin tendere "to stretch," tenuis "thin, rare, fine;" Old Church Slavonic tento "cord;" Old English þynne "thin"). Connecting notion between "stretch" and "hold" is "cause to maintain." The modern sense is probably because tenet was used in Medieval Latin to introduce a statement of doctrine.