Noun - the once sticky, now dried residue of beach sex used for the construction of sand sculptures.
'Dave, how come your sandcastle is so sturdy?'
'Well, Bob, funny you should mention come. The secret is Sandcastle Ejaculation .'
'Damn Jeff, that is one big fucking sandcastle'
'Yeah, my balls ache...I sure needed a lot of Sandcastle Ejaculate to make that tower.'