Latin for “in a class of its own”, as opposed to “ejusdem generis”, denoting something of the same kind as others.
By way of example (highlighted in bold): Inland Revenue Commissioners v Whitworth Park Coal Co Ltd  Ch 792, 815 per Jenkins LJ:
“(1) To come within the rule as an ‘other annual payment’ the payment in question must be ejusdem generis with the specific instances given in the shape of interest of money and annuities: see Hill v Gregory  2 KB 61, 70, 71 per Hamilton J and Howe (Earl) v Inland Revenue Commissioners  2 KB 336, 352, 353 per Scrutton LJ. Mr Bucher submitted that this requirement could not well be fulfilled by the interim income here in question consistently with Lord Radcliffe’s description of it in the Butterley Co’s case  AC 32 as ‘sui generis’. He said in effect that if interim income under the Coal Acts, 1946 and 1949, was in truth ‘sui generis’ it could not be ‘ejusdem generis’ with any other form of payment, and that the natural home of ‘sui generis’ income was Case VI of Schedule D. We can attach no great weight to this line of argument, for Lord Radcliffe was, as we think, directing himself to the peculiarity of interim income as income not arising from any trade or business or from any contractual obligation or from any income-bearing asset, but payable simply because (as he put it, at p 58) ‘the nationalization statute decreed that’ it ‘should be paid.’ The fact that it is ‘sui generis’ in these respects does not appear to us to preclude its inclusion in rule 1 (a) of Case III if it possesses the essential characteristics on which the application of the rule depends, which are in effect those appearing from the further propositions stated below.”