It means play loudly then suddenly softly. The "loudly" is somewhat less emphatically so then in the case of the related sfz (sforzando) - a sudden emphasis on a note/chord followed by the current dynamic, or the sforzando piano (sfzp or sfp) where there is a sudden emphasis on a note followed by piano.
It means that the f is original, but the fp is editorial.
The funny thing is that this would be an editorial of an editorial- Bach could never write dynamics; there is no dynamic variance with a harpsichord. So it was an older edition with F written in by an editor, then another editor treating the source like the manuscript but putting the P in there as well.
[...]All examples cited thus far stand as witnesses to an important observation regardingthe AMA: in Example 6.C the AMA excludes all dynamic markings from the text,even though it employs an additional, separate stave for the right-hand part of the firstedition’s text, and could thus enter the dynamics in brackets, complying with itsalleged editorial norms. But this was not always the case: in another instance(example 6.A), the editors of the AMA adopt an entirely different approach, byindicating the performance directions of the first edition in brackets or parentheses,while those originating from the autographs are marked without any brackets as partof the main text; in yet another instance (Example 6.D), the AMA entirely omitsdynamic marks that are contradictory between the autograph and the first edition(autograph marks for: while first edition marks pia:), leaving the passage spare ofdynamics, except for the movement’s final p.[...]
Is that generally true? If so, then just about anything goes I guess.