That sign is said to have to do with the fable of Kiyomasa KATO subjugating a tiger (there are other ways to say it including a sign for 'Ka' of the finger alphabet used with a sign to show 'to' [Japanese wisteria], a plant, and a sign imitating a mustache with one finger, which has to do with Cha KATO.)
これは加藤清正の虎退治の古事にちなんだものと言われる（他に指文字「か」＋植物の藤を表す手話単語を用いる場合や、タレントの加藤茶にちなんでつけ髭を指で真似る手話単語など）。