Be careful not to confuse Tung oil with the rarer and more expensive Tongue Oil. While I have never known Tongue Oil to go rancid, its "manufacture" can vary in quality affecting its odor.
It is an ages old product going back to the dawn of woodworking. Like its companion product, nose oil, it is collected by putting the tongue (or proboscus) in a tongue press for several hours to extract the oils. The procedure is quite uncomfortable, yet lucrative for the donor. The oils cure with exposure to air, a fact we have all unwittingly experienced when we sleep with out mouths open for too long. That yucky coating on your tongue in the morning is actually Tongue Varnish, not fully cured yet of course, fortunately for us mouth breathers.
Tongue oil odors are caused by the diets and mouth hygiene of those who are subjected to the tongue presses and can vary considerably as you might imagine.
Check the spelling on the bottle.
Ballard (Seattle), WA and Volcano, HI