It isn't "Leif" as in "life" or "Leif" as in "leaf" or even "Leif" as in "laugh." It's "Leif" as in "lafe," and why shouldn't he have his own CBS special? After all, Leif Garrett is 17 years old once had a hit single on the charts, and makes girls wail and squeal when he dances around in what look like Sandra Dee's old black pedal pushers.

The high point of "Leif," at 8 tonight on Channel 9, occurs late in the show as Garrett sings "When I Think of You" to the close-up faces of Brooke Shields and Marie Osmond. So much youth on one little screen! It's a convention of peach flesh.

Garrett can't sing a note that isn't lip-synched and over-dubbed, and his hair looks like someone poured a bowl of fettucini Alfredo over his head, but he is not without talent. He did a respectable job as the hero of an NBC Western "Peter Lundy," and appeared in a short-lived CBS series called "Three for the Road" in 1975. A clip from that is shown on the program, as is his unsuccessful screen test for the unsuccessful movie, "Mame."

Producer-director Bob Henry put "Leif" together with a diverting amount of dazzle, and some razzle as well, although adding Bob Hope to the Guest list only makes Hope look hopelessly ancient. Osmond proves pleasantly brassy when removed from the stifling environment of her deodorized brothers, and Shields remains an astonishing vision. Adults forced to watch "Leif" by their kids may be taken aback, but they won't be bored.