Chess Opening Theory/1. e4/1...e5/2. Nf3/2...Nc6/3. d4/3...exd4/4. Ng5

This line is dubious but sharp and can score well at low level.^[1]

If 4...h6, White plans to sacrifice their knight and draw out Black's king: 5. Nxf7? Kxf7 6. Bc4+, where they hope for the reasonable-looking 6...Ke8??, which leads to mate in six (7. Qh5+ g6 8. Qxg6+ Ke7 9. Qf7+ Kd6 10. Bf4+ Ne5 11. Qd5+ Ke7 12. Qxe5#). 6...Ke7 avoids this: Black's king is unsafe but they are up a knight for a pawn and should be winning, but the position is still very sharp.

4...Be7 is better, which attacks the misplaced knight on g5. If White continues with the same plan, 5. Nxf7? Kxf7 6. Bc4+, Black's king can step back to square vacated by the bishop, 6...Kf8!. If 7. Qf3+ Nf6 and Black's king is safe.

4...d5 is an alternative. This controls the c4 square and prevents White's bishop from coming there. If 5. exd5 Qxd5! and White doesn't have any attack left.

This line was first played in 1871 at the 2nd American Chess Congress by Arthur Johnston against Henry Hosmer.^[2][3] The game continued 4...Nh6 5. Bc4 Bc5 6. Qh5 Qf6.

Grandmaster Igor Smirnov suggested this line for blitz and bullet and proposed the name haggis gambit.^[4][5]