One of the most common (English) names for the contrabass, is 'Double Bass'. I can understand the other names for it, like 'Upright bass', 'Acoustic String Bass' etc, but what is double about it and is known as 'Double Bass' ?
The verb "to double" means that two instruments or voices play parallel lines of music, not necessarily at the same pitch - for example doubling in thirds, fifths, sixths, or octaves.
In instrument names, the adjective "double" usually means the pitch of the instrument is an octave below the "normal size." For wind instruments, the instrument is also double the length of the normal sized instrument - for example "double bassoon." "Double" is also used for organ stops that sound an octave lower than the usual pitch - http://www.organstops.org/d/_IndexN.html gives nearly 30 examples.
The Italian-based English names of orchestral string instruments are somewhat chaotic - a "violin" is literally a "little viol" and "violoncello" a "little big viol," neither of which is an accurate description. The original use of the double bass in the orchestra was to play the bass line an octave below the cello, though later it was often given an independent part.
The prefix "Contra-" seems to be replacing "Double" in both instrument and organ-stop names, especially in American English.
The "double" in double bass is due to its range compared to the cello which is an octave lower so it's "double the bass" compared to the cello.
The names contrabass and double bass refer to the instrument's range and use in the contra octave below the cello, also called the 16' octave relative to the church organ. --Source