The word whatsoever is an old-fashioned way to emphasize a point. If you have no clue whatsoever who ate all the ice cream that was in the freezer, it means you truly have no idea at all who the culprit is.

If you say that you have no doubt whatsoever that your cousin could be a model, you are placing emphasis on the fact that you're sure she's tall and thin enough to do the job. Whatsoever, originally quuat-so-euere, was first used in the thirteenth century, and while it's rare these days, it eventually led to the more modern and extremely popular word whatever.

Definitions of whatsoever
  1. adjective
    one or some or every or all without specification
    “no milk whatsoever is left”
    synonyms: any, whatever
    quantifier; used with either mass nouns or plural count nouns to indicate an unspecified number or quantity
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