[. . .]
2 freq attrib: An informal social gathering that guests may join and leave at any time during a set period. scattered, but esp PA, SC
1916 Phi Gamma Delta 39.282 ceOH, After the Cincinnati and Wilmington football games, drop-in parties have been held at the house. Both have proved quite popular with the coeds and it is quite likely that drop-ins after the games will be an established custom. 1932 Lancaster New Era (PA) 31 Dec /2, The events of the past week have kept the members of the younger set on their toes dashing from informal visits to teas and to drop-in parties to meet house guests, dinners in honor of those same guests and finally dances and more informal drop-in parties in the evenings. 1945 Columbia Rec. (SC) 4 Dec sec B 1/1, Mrs. Joseph L. Nettles is entertaining the debutantes, guests, and their escorts at a drop-in honoring Dorothy Johnson and Betty Coit. 1951 Wilkes-Barre Rec. (PA) 14 May 15/7, Mrs. William Bray, Mrs. Paxton Cole and Mrs. Walter Stevens will be hostesses at the drop-in party to be held Tuesday night at American Legion Home. 1958 Spokesman–Rev. (Spokane WA) 5 Sept 27/2, A drop-in membership brunch is planned by the auxiliary to the Spokane County Bar association. 1969 DARE File TN, OK, Drop-in—a party at which guests may “drop in” and leave at any time within stated limits; [held] any time of year. “I went to five drop-ins yesterday.” 1971 Charlotte Observer (NC) 28 July sec B 4/4, Miss Nancy Gurley, Aug 15 bride-elect of Walter Walker, was honored at a miscellaneous drop-in shower Tuesday. 1985 Times & Democrat (Orangeburg SC) 22 Dec sec C 9/3, Mr. and Mrs. J. R. Laird Sr. of North celebrated their 60th wedding anniversary on Dec. 1 with a drop-in given by their children. 2002 Sun. News (Lancaster PA) 24 Mar sec G 9/5, Joseph Sr. and Blanche Bryson . . celebrated their 50th wedding anniversary at a drop-in party held Sunday. 2012 State (Columbia SC) 29 July sec E 5/3, Jim Marion Cherry Jr., and Rosa Waring Thornton Cherry . . celebrated their 50th wedding anniversary . . with a group of family and friends at a Drop-In Brunch at the Florence Country Club.