Interesting. Even as a 41 year old native English speaker, I have never heard 'hold your tassels'. I know this phrase (assuming it is a variant of the same one) as 'hold your horses'. Hold your horses is still in fairly common use I would say. Certainly its origins are old fashioned - from the days when most people would have travelled by horse and cart/carriage. I have heard it relatively often, usually spoken in a mock Cockney accent for emphasis. "Owd yer 'orses mate!" I'm not sure that it would be a good idea for those unfamiliar with Cockney to imitate this though, it might make people stare at you in amazement if the rest of your English doesn't come up to confident use of local slang…