Ꝥ (minuscule: ꝥ), or Þ (thorn) with stroke was a scribal abbreviation common in the Middle Ages. It was used for (Modern English "that"), as well as , the / in ,AM 655, p1 recto, lines 4, 14, & 17 , , and . In Old English texts, the stroke tended to be more slanted, while in Old Norse texts it was straight. In Middle English times, the ascender of the þ was reduced (making it similar to the Old English letter Wynn (ƿ), which caused the thorn with stroke abbreviation ( ) to be replaced with a thorn with a small t above the letter ( ).
Unicode encodes Ꝥ as , and ꝥ at .
A thorn with a stroke on the descender also exists, used historically as an abbreviation for the word "through". The codepoints are , and .
References
Andrew West, What's that? , an article about the proposal to add medievalist characters to the UCS Unicode Character 'Latin capital letter thorn with stroke' (U+A764) Unicode Character 'Latin small letter thorn with stroke' (U+A765)