Comes from the Japanese counting system. The characters are each associated with a number.

So Hibari (or just Hiba for short) would be

Hi = 1
ba = 8

Making him "18". Yamamoto is "80" and Gokudera is "59", so "8059" is a YamamotoxGokudera pairing.



Here's a list:

G = Giotto
27 = Tsunayoshi Sawada
59 = Gokudera Hayato
80 = Yamamoto Takeshi
18 = Hibari Kyouya
L = Lambo
84 = Basil
D/Boss = Dino
594 = Kokuyo
493/193 = Chikusa
69 = Mukuro
96 = Dokuro
R = Reborn
56/C = Colonello
3/X = Xanxus
39/S = Squalo
100 = Byakuran
51 = Irie Shouichi
48 = Spanner
86 = Haru
87 = Hana

(Ten Years Later)
1027 = TYL Tsuna
1059 = TYL Goku
1080 = TYL Yamamoto
1069 = TYL Mukuro
1018 = TYL Hibari

Yesh I was also confused with those "Definitely 8059 and 1827 x3" comments xD
So Kimi though it would be helpful to put it here :3
(Taken from Yahoo Q&A) xD