This shirt is a particular piece of clothing worn by Benedict Cumberbatch's adaptation of Sherlock Holmes. The shirt is a piece that looks particularly good on him, leading many of the fandom to believe it's his "pulling" shirt - ergo; when he wears it when he wants/ is likely to get sex (usually from/ with his friend John Watson).
"Look, it's the Purple Shirt of Sex! Watson's totally checking him out."
by S. Moffat January 11, 2012