We get its current meaning naturally from combination of 士 and 心. However is this really how 志 was created long time ago? Let us examine its evolution history: (image taken from http://vividict.com)
|Learn one Chinese Character a day - 志|
From Oracle Script of 志 -, we can actually see that it was formed with (which is 之) and (which is 心). Now its meaning of 志 is even clearer - where your heart is. That is the precise meaning of 志. Now you will not be surprised that 志 (pronounces zhì) sounds very similar to 之 (pronounces zhī).
With the understanding of 志, now let us enjoy a Chinese calligraphy with 志 inside: