login

First recorded in 1965–70

Many who are neither professionals in the computer field nor amateur tech enthusiasts condemn the use of the solid form login as a verb, and with reason. It doesn’t behave like a normal verb. You cannot say you have loginned, and you are never in the process of loginning. Moreover, you cannot even ask someone to login you; you must ask that person to log you in. Clearly, it is the two-word phrase log in that functions fully as an English verb and not the solid form. Normally, we would expect log in, the verb phrase and login, the noun to behave in the same way as similar pairs: blow out/blowout, crack down/crackdown, hang up/hangup, splash down/splashdown, turn off/turnoff, where the two-word phrase is a verb and the one-word form a noun.
And yet, this gluing together of terms like login, logon, backup, and setup as verbs is common, especially in writing about computers. Not for everyone, however. Some well-known software companies, for example, carefully maintain the distinction in their programs and documentation.
But habits are difficult to change. Those who react to the one-word verb as an error will probably have to get used to it, and those who use the one-word verb will have to recognize that others will see it and wince.