Head Over Heels in Puppy Love?
This chiefly British way of expressing admiration entered English in the mid-1500s. Fancy implies a strong liking for another person, though it's not as loaded with emotions as the word love. Speakers use the word like to represent this same feeling in American English. This sense of fancy is not to be confused with a different, more obscure sense: "animal breeder."
When a person fancies someone, the next step is flirting. Early senses of flirt have associations with the movements of a fan. In the 18th and 19th centuries, a woman was said to be flirting a fan if she waved it smartly. Before that, flirting carried the sense of teasing or mocking someone. The sense of flirting we know and love today, which entered English in the late 1700s, still contains elements of mocking and levity. If someone flirts with another, it doesn't have to mean anything, though over-analytical minds will read deeply into every sigh, wink, smile, and hair toss.
In the process of flirting, sweet nothings are often whispered. In the late 1500s nothings was used to describe something trivial that was spoken, often to a lover. Gay nothings or soft nothings could be uttered in intimate conversation. Since the turn of the 20th century, the preferred phrase has been sweet nothings. This preference is reflected in popular song names; there are no fewer than seven songs with "sweet nothing(s)" in the title, including the Velvet Underground's "Oh! Sweet Nuthin'" (1970) and Diana Ross' "Sweet Nothings" (1981).
After sweet nothings have been whispered, it's time to go head over heels. This disorienting phrase is a transposed version of the now-obsolete heels over head, which made its debut in English in the early 15th century. It wasn't until the late 18th century that one could fall head over heels. While this phrase described literal tumbling at first, by the mid-1800s the metaphorical sense of being so infatuated with someone that you feel like you're somersaulting or "falling" became popular. This image is also evoked in the phrase falling for someone.
If it were the first half of the 20th century, and both parties were head over heels for each other, they might decide to go steady. While this phrase has largely fallen out of use with today's youth in favor of terms like dating, seeing someone, and going out, parents and grandparents might throw out the phrase going steady in their stories to younger generations. (Note that going steady implies a level of seriousness and commitment that the terms of the younger generation do not.)
A couple that is in a relationship, or that is at least seeing each other, might be colloquially called an item, or less commonly, a thing. Both terms emerged in the second half of the 20th century and are still used today. An item usually describes a more serious relationship than a thing, though this is not always the case.
Puppy love has been around since the mid-1600s. Its meaning has remained unchanged since that time, though in the 1800s calf love was a popular alternative way to refer to the same concept. Both phrases describe the intense admiration young people feel when they form romantic attachments.
When puppy love exists between young lovers, they will most likely display lovey-dovey behavior toward each other. Lovey-dovey, since as early as the 18th century, has been used as a noun; diminutive rhyming animal nicknames, such as honey-bunny, are just as popular in English now as when they entered the language. A lovey-dovey couple might also be referred to by onlookers as love birds, continuing the animal metaphor so commonly used in the realm of love.
While there are currently only two answers if someone pops the question--"Yes" or "Well, this is awkward"--the question didn't always so narrowly refer to a marriage proposal. That sense only began in the 1800s, though this phrase has existed in English since the latter part of the 16th century. While current English speakers will probably be able to count the number of times they pop the question on one hand, our ancestors could pop any number of questions on various topics throughout their lifetimes.