All my Japanese language budo books are in my office, so a fuller reply will have to wait till I go and check, but I can answer your question to some extent now.
'Ik-ka-jo' simply means 'first item', or 'first point'. In the glossary referred to by Ubaldo, it is given as the first technique in Daito-ryu, but the Japanese characters are not given. One possibility is ‰ÓŹđ.
As someone else said, the 'kyo' of 'ik-kyo' means 'teaching' or 'religion'. When applied to aikido techniques, the meaning becomes somewhat artificial, but 'first teaching' seems a reasonable translation.
It should be understood that the Founder of aikido did not use any names for techniques. These were coined by the students in order to remember what they had been taught. It is plausible that ikkyo was preferred to ikkajo by members of the Aikikai. There is also a logical progression from 1-kyo to 4-kyo in the sense that the four forms can be done successively in one movement (from elbow, to base of thumb, to base of wrist, to nerve point in wrist).
P A Goldsbury