Die hard is a 7-cell methuselah (essentially a collision between a block and the traffic light sequence) that vanishes after 130 generations, which is conjectured to be the limit for vanishing patterns of 7 or fewer cells. Note that there is no limit for higher numbers of cells, as eight cells suffice to have a glider heading towards an arbitrarily distant blinker.
"Die hard" as a general term
Alternatively, "die hard" or "diehard" may refer to any methuselah that eventually vanishes. Soups in Conway's Game of Life lasting at least 500 generations before disappearing completely are reported by apgsearch versions v4.69 and above and referred to as "messless methuselahs" on Catagolue.
As of early 2020, the longest-lasting diehard found in an asymmetric soup (and fitting inside a 16×16 bounding box) is a 1217-tick "C1" soup found by Rob Liston on January 10, 2020. The longest-lasting symmetric diehard (fitting inside a 32×32 bounding box) is a 1638-tick "C4_1" soup found by carybe on March 29, 2019.
In other rules
In HighLife, despite the instability of the traffic light sequence, the die hard still works, disappearing after 119 generations.