what the fuck?

Also known as Fast Inverse Square Root.

MC uses the double-precision version.
The magic number is one more than Robertson's constant.

It is used for atan2, which has also been mapped here.

mappings/net/minecraft/util/math/MathUtils.mapping

@@ -3,6 +3,7 @@ CLASS none/or net/minecraft/util/math/MathUtils
 	FIELD b sinTable [F
 	FIELD c rand Ljava/util/Random;
 	FIELD d MULTIPLY_DE_BRUIJN_BIT_POSITION [I
+	FIELD e SMALLEST_FRACTION_FREE_DOUBLE D
 	FIELD f asinTable [D
 	FIELD g cosTable [D
 	METHOD a generateRandomUuid ()Ljava/util/UUID;
@@ -92,6 +93,9 @@ CLASS none/or net/minecraft/util/math/MathUtils
 		ARG 2 b
 	METHOD c floor (D)I
 		ARG 0 value
+	METHOD c atan2 (DD)D
+		ARG 0 y
+		ARG 1 x
 	METHOD c lerp (DDD)D
 		ARG 0 time
 		ARG 1 v0