Greeting or call of/for recognition by member of group and/or to alert other members of desire to participate in activities calling for discretion by small group of acquainted persons moving within large group (sometimes 100s to 1000s) of unknown and unaware people, some of whom may be members of law enforcement or otherwise not inclined to view alcohol and/or drug use favorably (i.e. people who might arrest you or report you if they knew of group members' activities and altered reality). Also used as an "all - clear" signal.
"Hootie - hoo, Hootie - hoo" and wait for response from other group member.
Drinking and partying at large event, i.e. after a college football game, in lift line at ski slope (Snowblind or can't see your Buds) or at outdoor concert or in other large group of people with festive atmosphere, one may identify membership in group or acknowledge other members of group to let them know of your presence and that all is good and the necessary materials of use at that time are in close proximity or that you are a friend in need.
First use as defined above -- West Virginia University c. 1983 - 1987 to identify like-minded individuals -- also during the same time frame when "couch burning" became popular after football games
Use of term was based upon episode of Gomer Pyle, U.S.M.C., in which Pyle survives in wilderness for several days, captures enemy troops during war games and signifies his presence and alerts his side of his presence when returning to camp with prisoners by giving a "Hootie-hoo, Hootie-hoo" greeting.