Ꝥ (minuscule: ꝥ), or Þ (thorn) with stroke was a scribal abbreviation common in the Middle Ages. It was used for Old English "þæt" (Modern English "that"), as well as Old Norse "þor-", the "-þan"/"-ðan" in síðan, "þat", "þæt", and "þess". 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 U+A764 Ꝥ Latin Capital Letter Thorn With Stroke (HTML Ꝥ), and ꝥ at U+A765 ꝥ Latin Small Letter Thorn With Stroke (HTML ꝥ).
A thorn with a stroke on the descender also exists. The codepoints are U+A766 Ꝧ LATIN CAPITAL LETTER THORN WITH STROKE THROUGH DESCENDER (HTML Ꝧ), and U+A767 ꝧ LATIN SMALL LETTER THORN WITH STROKE THROUGH DESCENDER (HTML ꝧ).