-ght

The quick answer is that words in English ending in -ght fall into three groups: (1) verbal forms where the -t is a suffix in Old English denoting tense/aspect, (2) words that go back to PIE roots ending in -k(w)-t, and (3) miscellaneous.

In group 1, you have words like bought (< buy), brought (from bring), sight (from see), wrought (from work), etc. In group 2, words like night (cf. Latin nox, noctis, Greek nuks, nuktos), wight (and aught, nought, too, < PIE *wekti- 'being', cf. Russian veshch 'thing'). In group three, some words with unknown etymologies, e.g., blight. (There is some overlap between groups 1 and 2 because, it may be the same process but simply a matter of when it happened in the Old English, Proto-Germanic, or PIE period.) The main reason for supposing a suffix PIE -t- is that PIE roots are usually of the form CVC (simplified, with V covering a lot of other kinds of sounds than vowels). You can see this in the examples light and night above. Cf. Latin lux, lucis, 'light' which does not have the -t-, but German Licht, English light, which do.

Ceci n'est pas un seing.