Merriam-Webster's Collegiate® Dictionary, 11th Edition
I. transitive verb (-tied; -tiing; also -ting)
Date: 1964
to draw graffiti on ; to deface with graffiti ,
II. noun
Etymology: Italian, plural of graffito
Date: 1945
usually unauthorized writing or drawing on a public surface,
Usage: Graffiti, which also serves as the plural of graffito, is commonly used as a singular mass noun . This use is well established although not yet as well established as the mass- noun use of data. Use of graffiti as a singular count noun is still quite rare and is not standard.
Французско-русский словарь
m pl
1) графф'ити;
н'адписи, рис'унки ( на античных памятниках )
2) н'адписи
на ст'енах