"I'm too young to die," boasts Jack Nicholson in the course of his new movie, "Goin' South." That's what he thinks.

Exhibit A is none other than deadly ole "Goin' South" itself. This stupefying throwback - a bucolic sex comedy in which Nicholson the director indulges Nicholson the star an orgy of coy monkey-shines in the role of a scruffy outlaw who enters into a marriage of convenience with a demure young woman who owns a ranch and a goldmine - expires right before your eyes from a terminal case of the feebles.

It never occurred to me that the antique "close your eyes and think of England" joke would be revived. But there it is in "Goin' South," adapted to the Texas setting, circa 1866. Nicholson's virginal missus, played by a squirrely young actress named Mary Steenburgen, is taken aside by the womenfolk and advised to "think about canning apricots" if her spouse's sexual demands get a little insistent.

For the benefit of slow learners, Nicholson strolls in a few moments later to ask, "What do you say we try canning some of them apricots?" The next day when they take the buckboard into town, the incorrigible rascal teases his still untouched bride by shouting, "I sure did enjoy them canned apricots!" on main street.

"Goin' South" never surpasses this barren summit of verbal wit. As for sight gags, well, everything after the scene where Nicholson dumps a bucket of horse urine on the sheriff pales by comparison, including the humdinger where he tries a little bondage on his stuck-up spouse and darned if she doesn't kind of go for it. Ain't it funny the way some old jokes just keep getting older?

Nicholson's embarrassment, now at several area theaters, might justify a quick look if you find it instructive to observe how good performers can sometimes function as their own worst enemies. Mere fun-seekers should consider themselves adequately warned.

At some point another director would have said "Enough!" and advised him to put a lid on all the smirking, winking, strutting, grinning, leering, tongue-wagging, squinting, growling, cackling and scratching calculated to depict adorable rascality.

"Goin' South" is the most flat-footed comedy to collapse on the screen since "Nickelodeon." Why is Nicholson so slow on the uptake? Has his own reaction time slowed down or is he just underestimating the mental processes of the audience? Most of his lines are spoken with a peculiar cold in the nodze.

As a romantic comedy team Nicholson and Steenburgen prove such a bust that one almost feels mean for finding fault with relatively presentable mismatches like Jeff Bridges and Farrah Fawcett-Majors or George Segai and Jacqueline Bisset. Steenburger looks vaguely interesting, but Nicholson fails to inspire moods or feelings from her that might begin to establish rapport with the audience. If anything, he exposes her to undue contempt by conceiving the heroine's role in such insulting, reactionary comedy terms.

Do not place hope in the presence of John Belushi, who has perhaps two or three minutes of screen exposure in the role of a Mexican deputy. Nicholson is adding corroboration to the legend that winning an Academy Award can be jinx. Since "One Flew Over the Cuckoo's Nest" he has played second fiddle to Brando in "The Missouri Breaks" and fouled his own nest in "Goin' South." It's time to clean up the act and get back to serious acting.